a. Field of the Invention
The present invention relates in general to weather instruments, and in particular, to instruments which indicate the temperature of the dew point, defined as the temperature to which air, or any gas, must be cooled at constant pressure so that it will be saturated with respect to a vapor, in particular, water.
b. Prior Art
Dew point is important in aviation because it indicates the atmospheric temperature at which fog may be expected. Previously, dew point has been measured by (a) measuring both wet bulb and dry bulb temperatures for ambient air and obtaining dew point from a chart or (b) measuring relative humidity and ambient temperature and obtaining dew point from a similar chart.
One known dew point instrument is a dew hygrometer in which dew point is determined from heat flow into, and temperature of, a hygroscopic material, a lithium chloride moisture sensor, which is brought to vapor-pressure equilibrium with air.
Although there appear to be few dew point instruments in the prior art, others have previously recognized the association of humidity and temperature in determining human comfort. Many prior patents show comfort gauges, such as U.S. Pat. Nos. 3,709,039; 3,681,992 and 3,630,084. While the gauges have a utility of their own, they are not useful for measuring dew point.
O. F. Hevener, in U.S. Pat. No. 2,987,917, teaches use of a mechanical instrument in which relative humidity and temperature are elctro-mechanically balanced by a bridge circuit to produce an arithmetic means of the quantities. Hevener uses coiled elements for both temperature and relative humidity transducers. The instrument itself is a type of comfort gauge.
Perhaps one of the reasons that dew point instruments are not well known in the prior art is that there is a logrithmic relationship between dew point and relative humidity expressed by the equation EQU T.sub.D =T+(1/.alpha.) log RH (1)
where T.sub.D is the dew point temperature, T is ambient temperature, .alpha. is the average slope of a plot of the logarithm of the saturation vapor pressure of moisture in air versus ambient temperature, and RH is relative humidity. Since RH is less than or equal to one, the logarithmic term subtracts the required temperature difference from ambient temperature.
While good transducers for relative humidity and ambient temperature are known in the prior art, for example as taught by Hevener, there is a problem in building mechanical dew point gauges because of the logarithmic function of the relative humidity required in equation (1).
An object of the invention was to provide an instrument which gives a continuous reading of dew point temperature, which is relatively simple and compact, so that it can be used in general aviation, as well as elsewhere.